The present invention relates to a balun circuit, in particular, relates to such a circuit which is produced on an MMIC (Monolithic Micro-wave Integrated Circuit), and operates at frequency equal to or higher than 1 GHz.
A balun circuit is used for dividing and/or combining signals with the same amplitude and opposite phase with each other in a balanced frequency mixer.
A balun circuit is simple in structure as it comprises only a plurality of quarter wavelength coupled lines. The characteristic of a balun circuit depends upon characteristic impedance difference and phase velocity difference of even- and odd-modes. The larger the ratio of the characteristic impedance between even mode and odd mode is, and the smaller the phase velocity difference between even mode and odd mode is, the wider an operational frequency band of a balun circuit is.
As the phase velocity of even- and odd-modes of a coupled line differs with each other in an MMIC circuit, a prior effort to provide a wide band balun circuit has been directed to provide larger ratio of characteristic impedance between even- and odd-modes.
However, when we try to provide large ratio of characteristic impedance in a prior coupled line, size of the circuit must be large. Further, when we try to provide small phase velocity difference, the operational frequency band must be narrow.
Therefore, a balun cirucit having small size and wide operational frequency band has been desired.
FIG. 23 shows a prior balun circuit which is called a Merchand balun circuit. FIG. 23(A) shows an equivalent circuit of a balun circuit, FIG. 23(B) shows a cross section of a coupled line, and FIG. 23(C) shows an equivalent circuit of a coupled line. This structure is described in 1994 IEEE MTT-S International Microwave Symposium Digest, pp.389-391, by R. Schwindt.
In FIG. 23(B), the numeral 100 is a substrate made of GaAs which has a first surface on which a first conductor 106 and an insulation layer 102 made of SiO.sub.2 are deposited, and a second surface on which a ground metal 104 is deposited. A second conductor 108 is deposited on the insulation layer 102 so that the second conductor faces with the first conductor. The length of the first conductor 106 and the second conductor 108 is quarter wavelength. The width of the first conductor 106 is for example 750 .mu.m and the width of the second conductor 108 is for example 25 .mu.m so that the large characteristic impedance ratio between even- and odd-modes is obtained, and the typical thicknesses of the substrate 100 and the insulation layer 102 are 125 .mu.m and 0.75 .mu.m, respectively.
FIG. 23(C) shows an equivalent circuit of a coupled line which has a pair of parallel lines (a) and (b), which relates to the first conductor 106 and the second conductor 108 in FIG. 23(B). When a first end of the first line (a) is called an input port which accepts an input signal, the other end of the first line (a) is a through port to which an input signal passes, a first end of the second line (b) incorporated with the input port is a coupled port, and the other end of the second line (b) is an isolation port to which an input signal is not output.
A balun cirucit has a pair of coupled lines. In FIG. 23(A), a balun circuit has a first coupled line 1 which has the ports A, B, C and D, and a second coupled line 2 which has the ports A', B', C' and D'.
The first port B of the first coupled line 1 is connected to the first port A' of the second coupled line 2, the isolation port C when the first port B is an input port is grounded, the isolation port D' of the second coupled line 2 when the first port A' is an input port is grounded, and the through port B' of the second coupled line 2 is open.
With the above structure in FIG. 23(A), when an input signal is applied to the port P.sub.1 (port A) which is the through port when the first port B is an input port in the first coupled line 1, a pair of output signals of opposite phase are obtained at the ports P.sub.2 and P.sub.3 (port D and port C), which are a coupled port D when the port B is an input port, and a coupled port C' when the port A' of the second coupled line 2 is an input port.
FIG. 24 shows the explanatory curves of voltage standing wave V and current standing wave I along a half wavelength line between A and B' in FIG. 23(A). The current I is the maximum and the voltage V is zero at the center port B(=A') which is quarter wavelength from the input port A. The phase of the voltage V between the ports A and B(A') is opposite to that between the ports B(A') and B'. The amplitude of the voltage V is symmetrical concerning the center port B(A').
The phases at the ports D and C' which are coupled ports of the ports B and C' are opposite to each other.
Therefore, an input signal applied to the port 1 (A) is output to the output ports 2 and 3 with opposite phase and the same amplitude to each other.
FIGS. 25 and 26 show calculated characteristics of a balun circuit of FIG. 23, wherein FIG. 25 shows amplitude characteristics and FIG. 26 shows phase characteristics. A thick solid lines B, B.sub.1 and B.sub.2 (B.sub.1 is an outut at the port 2 and B.sub.2 is an output at the port 3) show the characteristics of a prior art of FIG. 23, and a thin solid line A shows an ideal characteristics. The parameters used in the calculation are as follows. The calculated results coincides well with the measured results.
(1) parameter of a coupled line of FIG. 23 PA0 (2) parameter of an ideal line (no loss line)
Ze=121.OMEGA. characteristic impedance of even mode PA1 Zo=21.OMEGA. characteristic impedance of odd mode PA1 .epsilon..sub.e =3.02 effective dielectric constant of even mode PA1 .epsilon..sub.o =4.22 effective dielectric constant of odd mode PA1 .alpha..sub.e =0.15 dB/mm at 10 GHz loss of even mode PA1 .alpha..sub.o =0.60 dB/mm at 10 GHz loss of odd mode PA1 Ze=500.OMEGA. characteristic impedance of even mode PA1 Zo=21.OMEGA. characteristic impedance of odd mode PA1 .epsilon..sub.e =3.02 effective dielectric constant of even mode PA1 .epsilon..sub.o =3.02 effective dielectric constant of odd mode
It should be noted in FIGS. 25 and 26 that the prior Marchand balun circuit of FIG. 23 has the disadvantage that the amplitude and the phase deviates much in the operational frequency band, and therefore, the operational frequency band is essentially narrow. It is preferable in practice that the phase difference in an operational frequency band is within 10.degree., and the amplitude deviation in an operational frequency band is within 1 dB.
The reason why the operational frequency band in a prior Marchand balun circuit using a micro-strip line MMIC, a coplanar wave-guide MMIC deposited on a semiconductor substrate of GaAs and Si, or a three-dimensionalal MMIC which has dielectric multi-layers on a semiconductor substrate, together with other active circuits like an FET and other passive circuits, is narrow, is that (1) an even mode characteristic impedance of a coupled line which constitutes a balun circuit is small and it can not be large on principle, (2) even- and odd-modes have phase difference, and (3) transmission loss of a coupled line which constitutes a balun circuit is larger (larger than 0.1 dB/mm) than that of a conventional wave-guide, or a conventional coaxial cable.
FIGS. 27 and 28 show another prior balun cirucit produced on an MMIC. FIG. 27 is described in IEEE Trans. on MTT-41, No12, pp. 2330-2335, December 1993, by S. A. Maas, and FIG. 28 is described in 1995 IEEE Micro-wave and Millimeter-wave Monolithic circuits Symposium Digest, pp.155-158, by M. I. Ryu.
In FIG. 27, FIG. 27(A) is an equivalent circuit of a balun circuit, and FIG. 27(B) is cross section of a coupled line of a balun circuit of FIG. 27(A). In FIG. 27(B), a coupled line is in interdigital type having a substrate 100 made of GaAs on which a ground conductor 98 and a plurality of coupling lines 99 are deposited. The thickness of the substrate 100 is for instance 635 .mu.m.
A coupled line 130, 140 of FIG. 27 has three fingers, and a coupled line 7, 8 of FIG. 28 has seven fingers.
The structure of FIGS. 27 and 28 has the advantage that the even mode characteristic impedance is large, and the phase velocity difference between even- and odd-modes is small, thus, an excellent balun is obtained.
However, the structure of FIGS. 27 and 28 has the disadvantage that the width of the circuit is large because of many fingers, and the thickness of the substrate is large, thus, the size of a circuit can not be small. Further, the operational frequency band of FIGS. 27 and 28 is smaller than that of FIG. 23.